1. Field of the Invention
The present invention relates to graded-index rod lenses used in image writing devices and image reading devices, and methods for manufacturing the same, and in particular relates to the compositions of glass covering the lateral surface of the lens when it is manufactured.
2. Description of the Related Art
Graded-index rod lenses are rod-shaped lenses with a cross-sectional refractive index distribution that changes from the center toward the circumference. Because of the many advantages of graded-index rod lenses, such as their ability to form images even with flat end faces and the ease with which they can be used to fabricate lenses with miniature diameters, in recent years they have found wide application as image-forming lenses in the optical systems of image writing devices such as LED array printers and liquid crystal shutter printers and image reading devices such as fax machines and scanners.
Such graded-index lenses having this wide range of application are fabricated by forming a graded refractive index distribution within a glass body from its center toward its circumference through cation exchange. With cation exchange, a glass body including first cations that can be used as network modifier oxides is brought into contact at elevated temperatures with molten salt including second cations that can constitute network modifier oxides, thereby substituting the first cations in the glass body with the second cations in the molten salt.
The three following methods are usually performed to provide such graded-index rod lenses with a rod-shape.
(1) Shaving Method
A rod of a predetermined shape is produced by shaving a glass block. With this method, only a few rods can be produced at one time and it is difficult to produce rods with a rod diameter of 0.5 mm or less.
(2) Drawing Glass Method
Raw glass in block form is processed, producing a preform rod with a diameter of approximately 20 to 50 mm and a length of approximately 200 to 800 mm, and this is suspended in a tubular furnace and elongated as it is heated, obtaining a glass rod. This method offers a significant improvement in productivity over the shaving method and can be used to produce rods with narrow diameters. However, although devitrification is unlikely to occur, this method is not suited for large-scale production because the drawing velocity is slow at about 1 m/min and it is necessary to perform drawing for each rod individually. Moreover, it is difficult to produce rods with a double core/clad structure, which will be described later.
(3) Direct Drawing Method of melt glass (Continuous Drawing Method)
As shown in FIG. 1, a molten raw glass 11 that has been subjected to melting, bubble removal, and a fining process is incubated by a heater 13 inside an insulating material 15, gradually cooling as it is allowed to flow down through a cylindrical nozzle 12. It is then extruded out through a nozzle lower end 14, and subjected to heating and stretching, forming a continuous glass rod (fiber) 17 having a diameter of about 0.1 to 4 mm. With this method, the drawing velocity can be set to a drawing velocity that is of a magnitude dozens of times that of the drawing glass rod method, and there is extremely high productivity because raw glass can be fed continuously, allowing uninterrupted production.
These three methods are chosen depending on the ease with which the rod can be formed.
With the direct drawing method of melt glass, devitrification occurs easily when the molten glass is slowly cooled within the nozzle. In particular, to prevent devitrification in the preform glass for graded-index lenses including a large lithium component, measures such as adding an additive have been taken (see JP H8(1997)-13691B).
As shown in FIG. 2, direct drawing allows a rod with a double structure, in which the lens preform is covered by providing a two-channel nozzle, to be provided easily. If the composition of the clad glass of the double structure is a composition with which devitrification does not occur easily, then with direct drawing, contact between the lens preform glass and the nozzle portion in the temperature range where devitrification occurs easily can be avoided, and thus devitrification in the lens preform can be prevented and the overall drawing properties are improved (see JP H10(1998)-139468A).
Providing the rod with a double structure for example makes it possible to add the effect of stray light removal if a color component is introduced into the portion with the clad glass composition and a plurality of rod lenses are used arranged in a line, and also allows cracks, which occur easily due to the stress that occurs in the lens preform portion during ion exchange, to be inhibited. Thus compared to a single structure, a double structure often allows the rod to be provided with superior characteristics (for example, see JP H10(1998)-139472A).
Thus, direct drawing is the method that usually is used to mass-produce high-performance rods having a double structure employing clad glass.
The graded-index rods that are the subject of this invention are those that as mentioned above are used in the optic systems of devices for reading or writing images. Consequently, particularly when handling full-color images, there is a need to simultaneously maintain a sufficient angle of aperture and keep the color dispersion small. To reduce color dispersion, a preform glass composition that does not include PbO as a component is preferable. However, in terms of manufacturing, there is the problem that a composition that does not include PbO is extremely prone to devitrification.